122 ORGANIC EVOLUTION CONSIDERED 



malial As to the Didelphia, if we may trust the evi- 

 dence which seems to be afforded by their very scanty 

 remains, a Hypsiprymnoid form existed at the 

 epoch of the Trias, contemporaneously with a car- 

 nivorous form. At the epoch of the Trias, therefore,, 

 the Marsupialia must have already existed long 

 enough to have become differentiated into carnivor- 

 ous and herbivorous forms. But the Monotremata 

 are lower forms than the Didelphia, which last are 

 intercalary between the Ornithodelphia and the 

 Monodelphia. To what point of the Paleozoic epoch, 

 then, must we, upon any rational estimate, relegate, 

 the origin of the Monotremata?" 



If the theory of evolution is true, then I have no 

 doubt that this argument is correct. Each order of 

 animals, when it first appears in the fossil form, is 

 highly developed, and therefore it must have been 

 preceded by a long ancestral line of animals leading 

 up to the fossil forms. 



The Eocene contains six of the orders of mono- 

 delphous mammals. If these highly differentiated 

 orders were evolved from a common stock it is evi- 

 dent that when they reached the Eocene they had 

 already had a long history as monodelphous mam- 

 mals. Prior to their history as Monodelphia they 

 must have existed for a long period as Didelphia and 

 before that as Monotremata. But we know that 

 there were a good many species of mammals in the 

 Jurassic, most of which are thought to have been 

 insectivorous marsupials, with probably some rodents. 

 In the Triassic several species of mammals have been 

 found. 



The existence, therefore, of so many kinds of mam- 

 mals in the Eocene, Jurassic, and especially Triassic, 

 shows beyond doubt that if mammals were evolved 

 they must have. begun as mammals at some point in 



